Can CGI elements emit light when in real video?
You would camera project parts of your video to objects within the Blender scene and those objects catch the surrounding light. Than you would comp it over your footage.

here is a small YT Video that shows the process with an Image but you can also use Video or Image Frames - but be aware that you have a good tracking or you will get sliding effects  

https://www.youtube.com/watch?v=p9IfDeNU8K0

ohhh and for sure
BlenderGuru's Camera Mapping tutorial  

https://www.youtube.com/watch?v=B9r9tKX5olY

if you set up your scene like this, you can have your NeonSign lighten Up the surrounding Areas.